1. Field of the Invention
The present invention relates to communication devices, and more particularly, to a communication device incorporated in a base station for allocating one of a plurality of frequency bands to a terminal to enable wireless communication.
2. Description of the Related Art
In cellular mobile communications system, mobile terminals communicate with a base station on radio channels. As a terminal moves during communication, switchover from one base station to another may take place to keep the wireless communication alive. This switchover operation is called a “handover” in cellular terminology.
Referring to FIG. 26 showing a mobile communication system, an example of handover will be explained. This figure shows base stations (BSs) 101 and 102, a terminal 111, and cells 121 and 122 representing the coverage of the respective base stations 101 and 102.
The terminal 111 receives common pilots from the base stations 101 and 102, and compares the received power levels to determine whether to ask for a handover (HO).
As the terminal 111 moves from the current cell 121 toward its neighboring cell 122 as indicated by the arrow in FIG. 26, the terminal 111 receives a higher power level from the neighboring base station 102. At this time, the terminal 111 sends a handover request to the current base station 101 to initiate a handover to the base station 102.
FIG. 27 illustrates a handover operation in the mobile communication system. This operation is performed by the terminal 111 and the base stations 101 and 102, all appearing in FIG. 26, and a core network (CN).
Step S121: The terminal 111 in the cell 121 exchanges packets with the core network through the current base station 101. Using the general and common core network protocol, the terminal 111 could communicate with another terminal or the like (normal communication).
Step S122: Suppose that the terminal 111 moves into the cell 122. Consequently, the pilot power level of the base station 102 becomes higher than that of the base station 101, and thus the terminal 111 sends a handover request to the base station 101.
Step S123: The base station 101 transfers the received handover request to the base station 102 which is the target of handover.
Step S124: On receiving the handover request from the terminal 111, the base station 102 sends a switching request to the core network. Consequently, the core network switches the edge node from the base station 101 to the base station 102.
Step S125: The base station 101 transmits untransmitted packets remaining therein to the terminal 111.
Step S126: The core network, having received the switching request, starts to route packets destined for the terminal 111 to the base station 102.
Step S127: On completing the transmission of all the remaining packets to the terminal 111, the base station 101 sends a handover switching request to the terminal 111.
Step S128: The core network routes packets to the base station 102, if any.
Step S129: The terminal 111, having received the handover switching request transmits a packet send request to the base station 102.
Step S130: The core network keeps routing packets to the base station 102, if any. Those packets are accumulated in the base station 102.
Step S131: The base station 102, having received the send request, starts to transmit accumulated packets to the terminal 111.
Step S132: The terminal 111 exchanges packets with the core network through the base station 102. Using the general and common core network protocol, the terminal 111 could communicate with another terminal or the like (normal communication).
As described above, when the terminal 111 sends the handover request, the handover source base station 101 has remaining packets to be transmitted, and subsequent packets (packets in steps S126, S128, and S130) are transmitted and accumulated in the handover target base station 102.
FIG. 28 illustrates remaining packets and accumulated packets. In FIG. 28, like reference numerals are used to denote like elements appearing in FIG. 26, and description of such elements is omitted.
This figure shows a memory 131 storing packets remaining in the base station 101, a memory 132 storing packets accumulated in the base station 102, and a core network 141. At the time of a handover, the terminal 111 receives the remaining packets from the base station 101, and then the accumulated packets from the base station 102.
Since the terminal 111 sends a handover request when a power level from the base station 102 becomes higher than that from the base station 101, the terminal 111 has to receive the remaining packets from the base station 101 under interference of the cell 122 of the base station 102.
As stated above, a handover takes place around the boundary between cells, and therefore, the adjacent cell interferes with reception of remaining packets, which results in a lower throughput of the packets.
Meanwhile, a cellular system has been proposed in which a mobile station can transmit Acknowledge/Non-Acknowledge (ACK/NACK) signals through High-speed Dedicated Physical Control Channel (HS-DPCCH) at a low error rate and without packet loss even when it is connected to a plurality of base stations on Dedicated Physical Channel (DPCH) to achieve a soft handover (cf. Japanese Unexamined Patent Publication No. 2004-7030).